1. Field of the Invention
The present invention relates to display elements in which light emission is controlled by the amount of current flowing through an electro-optical element, such as an organic EL element or an inorganic EL element, as well as to display devices including such display elements.
2. Description of the Related Art
To date, there are display devices whose light emission sources are electro-optical elements whose optical characteristics are changed by applying electricity, such as organic EL (electroluminescent) elements, inorganic EL elements or light emitting diodes. In such display devices, a plurality of display elements each including an electro-optical element are arranged in a matrix, and the current necessary for lighting the electro-optical elements is supplied to the electro-optical elements from a power source supplying a predetermined current. These electro-optical elements are controlled to a predetermined luminance by applying a predetermined data signal voltage (or current).
Ordinarily, a display element includes a plurality of TFT (thin film transistor) elements, an electro-optical element, an auxiliary capacitance, a data signal line for applying data signals, a scanning signal line for applying scanning signals, and a power source line for supplying current from the power source. Here, the size (occupied surface area) of the electro-optical element in the display element is determined by the size (occupied surface area) of conductors and the elements other than the electro-optical element. For example, in order to make the numerical aperture of the pixels as large as possible, the size of the elements other than the electro-optical element should be made small and also the size of the conductors, in other words the width of the electrodes, should be small. If that is the case, then the numerical aperture can be made large, and the light emission efficiency of the electro-optical element increases, so that it is possible to drive with a comparatively low voltage. As a result, also the power consumption becomes relatively low.
However, the larger one tries to make the numerical aperture, the thinner the electrode width of the power source line needs to be made, so that the resistance of the power source line electrode increases. Consequently, when current is supplied from the power source line electrode to the display elements, the voltage drop at the display elements further away from the current supplying power source increases the greater the numerical aperture is made. This means that there are variations in the drop of the voltage applied by the current supplying power source to each of the display elements. When the voltage applied to the electro-optical element becomes small due to this voltage drop, then also the emitted luminance of the electro-optical element decreases. Therefore, the larger one tries to make the numerical aperture, the bigger are the variations that occur in the luminance of the pixels of the display device. Ordinarily, electro-optical elements such as organic EL elements have diode characteristics, so that the current flowing through the electro-optical element changes exponentially with changes in voltage. Thus, since the luminance of the electro-optical element is lowered exponentially due to the above-described voltage drop, the display device suffers from conspicuous variations or irregularities in luminance, lowering the display quality.
Conventionally, display devices address this problem by suppressing luminance variations on the display screen with the following configurations. For example, there are display devices that are provided with a memory for storing data expressing the image to be displayed and a memory for storing current correction data, which has been set in advance, for correcting the luminance variations, and image data that is corrected in accordance with the current correction data is applied to the display elements (see for example JP H11-344949A). With this configuration, luminance variations can be corrected. Moreover, there are display devices in which the conductors leading from a predetermined driving circuit to the display elements are provided with a predetermined resistance distribution (see for example JP 2001-83934A). This resistance distribution is set such that voltage drops due to the resistance of the conductors are suppressed. Thus, it is possible to reduce luminance variations due to voltage drops.
However, in the configuration disclosed in JP H11-344949A, a new correction circuit including a memory for storing the current correction data becomes necessary. When this correction circuit is provided in a driver portion incorporated in the display panel, the scale of the circuitry increases, which is undesirable. When the scale of the circuitry increases, there are disadvantages with regard to the manufacturing yield, for example. Moreover, with the configuration disclosed in JP 2001-83934A, the resistance distribution of the conductors must be set such that voltage drops due to the resistance of the conductors are suppressed, so that complex design and layout of the conductors becomes necessary.
To do so, the electrode width of the power source line should be increased in order to suppress luminance variations, but in this case, the numerical aperture is decreased, as mentioned above, worsening the light emission efficiency of the electro-optical elements. This causes the problems that the element lifetime is shortened, and the power consumption is increased.
Moreover, in conventional display devices, one pixel is configured by a plurality of sub-pixels, such as three sub-pixels for displaying the three colors for RGB display. In this case, the current for generating the necessary luminance depends on the (light emitting material of the) display elements forming the sub-pixels, so that when the luminance of the various colors deviates from the desired values when displaying white light for example, then a colored hue may be perceived on the display screen.